1. Field of the Invention
The present invention relates to a brushless DC fan motor suitable for a fan radiating heat from a housing of an electronic appliance, and more particularly to a speed control circuit thereof
2. Description of the Related Art
In electronic appliance such as OA appliances including personal computers and copiers, a large number of electronic components are accommodated in a small housing, whereby heat is generated from the electronic components and accumulated in the housing so as to have a possibility to damage the electronic components.
To solve this problem, a ventilation hole is provided in a wall or a ceiling of the housing, and a fan motor is mounted on the ventilation hole so as to radiate heat outside of the housing.
For using the brushless DC fan motor by making the temperature of the housing controlled, the speed of the motor should be accurately controlled.
FIG. 2 shows a speed control circuit of a conventional brushless DC fan motor meeting this request.
In the figure, numeral 21 denotes the speed control circuit of the brushless DC fan motor (circuit) 22. The two-phase motor 22 is employed here.
As shown in the figure, the brushless DC in motor 22 comprises field coils L1 and L2, switching elements SW1 and SW2, Zener diodes ZD1 and ZD2, resistors R1 to R4, a diode D1 and a drive circuit 22a. 
The field coils L1 and L2 are mounted on a stator (not shown) and electrified in an alternately switching manner to form the rotating magnetic field by the switching elements SW1 and SW2 alternately turning ON/OFF by the control signal from the drive circuit 22a. A rotor (not shown) is rotated by rotating a permanent magnet mounted thereon following the above rotating magnetic field.
The position signals S1 and S2 from a sensor to detect the position of rotation of the rotor are inputted in the drive circuit 22a as the timing signal of switching and electrifying the field coils L1 and L2.
The speed control circuit 21 comprises resistors R5 to R10 and an NPN transistor Q1. The signal (PWM signal) in which the temperature in a housing for an OA appliance (not shown) is Pulse-Width Modulated (PWM, i.e., the signal in which the temperature in the above housing is converted into the pulse width (time) ratio on the H (High) and L (Low) levels of the voltage is inputted as the input signal in an input terminal IN of the speed control circuit 21. The inverted signal of the above input signal is outputted from an output terminal OUT of the speed control circuit 21.
The speed control circuit 21 either validates or invalidates the control signal to the switching elements SW1 and SW2 of the motor 22 in the drive circuit 22a by the voltage level of the output signal.
More specifically, the speed control circuit 21 validates the above control signal when the voltage Vb of the output terminal OUT (a control input terminal 22b of the drive circuit 22a) is less than a predetermined value (the L-level). As a result, the switching elements SW1 and SW2 are alternately turned ON/OFF. When the voltage Vb is not less than the predetermined value (the H-level), the speed control circuit invalidates the above control signal, and turns off the switching elements SW1 and SW2.
The driving force is given to the rotor by alternately electrifying the field coils L1 and L2, or the driving force is not removed from the rotor by shutting off the electrification to the field coils L1 and L2, and the rotational speed of the motor 22 is thus controlled.
In such a speed control circuit 21, when the temperature in the housing increases and the H-level time of the PWM signal to be inputted in the input terminal IN is increased, the H-level time of the base voltage of the transistor Q1 also increases, and the L-level time of the voltage Vb of the output terminal OUT increases.
As a result, the L-level time of the input voltage in the drive circuit 22a, i.e., the time in which the above control signal is valid is also increased, the time in which the driving force is continuously given to the rotor (not shown) is increased, and the rotational speed of the motor is thus increased.
As a result, the heat radiation effect is improved so as to decrease the temperature in the above housing. Thus, if the PWM signal in which the temperature in the housing is pulse-width modulated is set to be the input signal of the speed control circuit 21, the drive circuit 22a can be finely controlled by the ratio of the H-level to the L-level, the speed of the motor 22 can be accurately controlled such that the temperature in the housing can be efficiently adjusted.
In the conventional speed control circuit 21 described above, the PWM signal has been used for the input signal for the follow reason. That is, an operational point is established on the voltage level in a rise characteristic curve of the transistor Q1 when the output voltage is taken out from a collector by using the voltage signal of the continuously changing level for the base input signal of the transistor Q1 like the temperature detecting output voltage from a thermistor.
However, the transistor Q1 has a steep but non-linear rise characteristic, and is difficult to set the above operational point. Thus, it is difficult to improve the accuracy in controlling the speed by either validating or invalidating the control signal to the switching elements SW1 and SW2 with a predetermined temperature (the voltage level of the input signal) as a reference.
On the other hand, when the PWM signal is used for the input signal, the temperature is expressed by the pulse width, i.e., the length on the time axis, and thus, the above operational point need not be finely set, and the setting thereof can be simplified. Accordingly, the conventional speed control circuit 21 has used the PWM signal for the input signal.
However, the conventional speed control circuit 21 requires an element to convert the original physical quantity used to control the speed, i.e., the temperature here into the electric signal, for example, an expensive oscillator for PWM to convert the electric signal into the electric signal in addition to the thermistor, and there occurs a problem, in that the cost required for a circuit of the prestage is considerably increased.